refactor: resolve function regions during build phase to fix VPC connectors

CHANGELOG.md

@@ -0,0 +1 @@
+- Resolves undefined regions earlier, during the build to backend resolution phase (#10471)

src/deploy/functions/build.ts

@@ -257,8 +257,8 @@
   // Defaults to the compute service account when a function is first created as a GCF gen 2 function.
   serviceAccount?: ServiceAccount | Expression<string> | null;
 
-  // defaults to ["us-central1"], overridable in firebase-tools with
-  //  process.env.FIREBASE_FUNCTIONS_DEFAULT_REGION
+  // Defaults to REGION_TBD. The deployment region is resolved dynamically at deploy-time
+  // based on event trigger sources or matching existing functions, falling back to "us-central1".
   region?: ListField;
 
   // The Cloud project associated with this endpoint.
@@ -483,7 +483,7 @@
       // List param, we try resolving a String param instead.
       try {
         regions = params.resolveList(bdEndpoint.region, paramValues);
       } catch (err: any) {
         if (err instanceof ExprParseError) {
           regions = [params.resolveString(bdEndpoint.region, paramValues)];
         } else {